During the past decade, broadband wireline and wireless mobile services have been two of the most remarkable growth areas in the telecommunications industry. Broadband access has been enabled by technologies such as Digital subscriber line (DSL), cable modems, and fiber-to-the-home (FTTH). Wireless mobile access has been driven by 2G and now 3G cellular systems. In addition to cellular systems, Wi-Fi systems based on the IEEE 802.11 family of standards, have become enormously popular for providing in-building wireless coverage. Wi-Fi systems offer much higher peak data rates than 3G systems, but are not designed to support high-speed mobility.
Worldwide Interoperability for Microwave Access (WiMAX) is an emerging technology based on the IEEE 802.16 family of standards for broadband wireless mobile access that provides a rich set of features and a high degree of flexibility. As used herein, the term “WiMAX” refers to “mobile WiMAX,” i.e., the WiMAX standard that accommodates mobile subscribers. Unlike 3G systems, which provide only a fixed channel bandwidth, WiMAX allows the user to select an adjustable channel bandwidth from 1.25 MHz to 20 MHz. By using Orthogonal Frequency Division Multiplexing (OFDM) as the primary modulation scheme, WiMAX, as well as Wi-Fi, is able to support much higher peak data rates than 3G systems that are based on Code Division Multiple Access (CDMA), which requires bandwidth spreading. OFDM is a multicarrier modulation scheme whereby a given high-rate data stream is divided into several parallel low bit-rate streams, each of which is modulated onto a separate carrier called a subcarrier or tone. The multiple access scheme adopted by WiMAX is Orthogonal Frequency Division Multiple Access (OFDMA), whereby the available subcarriers are further divided into groups called subchannels, which can be allocated to different users. In OFDMA, the subcarriers assigned to a subchannel need not be contiguous, allowing for a flexible assignment of data rates to users.
In recent years, dynamic spectrum access (DSA) has been an active area of research because of its potential to exploit highly underutilized wireless spectrum. Cognitive radios with frequency agility enable DSA by sensing spectrum “holes” and automatically tuning to available frequency channels. Much of the research on DSA has focused on systems consisting of secondary users equipped with cognitive radios that attempt to utilize spectrum that is not being used by the primary licensed users at a particular time and place.
WiMAX and Cellular Systems
The WiMAX Forum is promoting broadband wireless technology based on the IEEE 802.16 family of standards. The original 802.16 standard for fixed wireless access was completed in 2001 based on a single-carrier physical (PHY) layer with a medium access control (MAC) layer based on TDMA (time division multiple access). Subsequently, 802.16a was developed, based on orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA). Further revisions resulted in a new standard released in 2004 called IEEE 802.16-2004 (IEEE, “Standard 802.16-2004. Part 16: Air interface for fixed broadband wireless access systems” (October 2004)), which replaced all prior versions of 802.16. This standard formed the basis of the first WiMAX standard, referred to as fixed WiMAX. In 2005, a new standard called 802.16e-2005 (IEEE, “Standard 802.16e-2005. Amendment to IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems-Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands,” (December 2005)) was completed, which included mobility support. This standard forms the basis for mobile WiMAX technology. Although the examples and configurations described herein assume the use of mobile WiMAX, as used herein the term WiMAX refers to both fixed and mobile WiMAX.
A. WiMAX Physical Layer
The WiMAX PHY specifies several operational frequency bands, including 2-11 GHz for fixed applications. An earlier version of the 802.16 standard specified a frequency band of 10-60 GHz for fixed applications. The PHY layer is based on OFDM, a multicarrier modulation scheme which provides strong mitigation of multipath effects and allows for operation in non-line-of-sight (NLOS) conditions. In OFDM, a high bit-rate stream is divided into several parallel low bit-rate substreams, each of which is modulated onto a separate subcarrier or tone. The substream symbol time is chosen to be large enough so that the delay spread incurred by the wireless channel is a small fraction of the symbol duration, thus minimizing intersymbol interference (ISI). The subcarriers are chosen to be mutually orthogonal over the symbol period, such that the subcarrier channels need not be nonoverlapping.
An OFDM signal can be generated by taking the inverse Discrete Fourier Transform (IDFT) of the input data stream in blocks of L symbols, where L is the number of subcarriers. OFDM transmitters and receivers can be implemented with low complexity using the Fast Fourier Transform (FFT). Besides ISI-mitigation and low computational complexity, OFDM provides frequency diversity by allowing coding/interleaving across subcarriers and robustness against narrowband interference.
The multiple access technique used in WiMAX is called scalable OFDMA because the FFT size used in OFDM can be scaled from 128 to 2048. As the available spectrum bandwidth increases, the FFT size for OFDM can be increased to maintain a constant subcarrier spacing. Typically, the subcarrier spacing is 10.94 kHz. Thus, when the channel bandwidth is 1.25, 5, 10, and 20 MHz, the FFT size is set to 128, 512, 1024, and 2048, respectively, to maintain the 10.94 kHz subcarrier spacing.
In OFDMA, the available subcarriers are further partitioned into groups of subcarriers called subchannels. Different subchannels are assigned to different users. By assigning different numbers of subcarriers to subchannels, fine-grained resource allocation may be achieved. The subcarriers making up a subchannel need not be contiguous. Subchannels consisting of noncontiguous subcarriers offer greater frequency diversity. WiMAX defines several different subchannelization schemes for both the uplink and downlink.
OFDMA may be considered a hybrid of TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access), in the sense that users are allocated both OFDM subcarriers and time slots. OFDMA offers the multipath suppression and frequency diversity of OFDM plus flexible allocation of rates to users. In the time domain, data may be transmitted in the form of frames. The minimum time-frequency unit that can be allocated to a user is a slot. A slot consists of a subchannel over one, two, or three OFDM symbols, depending on the subchannelization scheme that is used. A contiguous set of slots assigned to a user is called a data region.
WiMAX supports both TDD (Time Division Duplexing) and FDD (Frequency Division Duplexing). FIGS. 1-2 shows sample WiMAX OFDMA frame structures for FDD and TDD. FIG. 1 shows a sample WiMAX FDD frame structure consisting of 6 data regions on the downlink and uplink, respectively. Similarly, FIG. 2 shows a WiMAX TDD frame structure with 6 data regions. In both cases, the uplink and downlink media access protocol (MAP) messages (UL-MAP and DL-MAP) specify the allocation of users to data regions within the frame. The ranging channel in the uplink portion of the WiMAX frame provides contention-based access for frequency, time, and power adjustments. The ranging channels can also be used by a mobile station (MS) to make uplink bandwidth requests. As used herein, the term subscriber station (SS) is equivalent to mobile station. Best-effort traffic may also be transmitted on the ranging channel when the amount of data to be sent is relatively small. The frame size can be varied on a frame-by-frame basis from 2 to 20 ms, but the nominal frame size is 5 ms. With an OFDM symbol duration of 102.9 μs, the number of OFDM symbols in a 5 ms frame is 48. In general, TDD allows for simpler and flexible sharing of bandwidth between uplink and downlink. On the other hand, TDD requires synchronization across multiple base stations.
B. WiMAX MAC Layer.
The WiMAX MAC layer takes packets from the upper layer, called MAC service data units (MSDUs) and transforms them into MAC protocol data units (MPDUs) for transmission over the PHY layer. The MAC layer includes a convergence sublayer that can provide an interface to various high layer protocols, but currently only IP and Ethernet are supported. In WiMAX, the base station is responsible for allocating bandwidth to all users on the uplink and downlink. On the downlink, the BS may allocate bandwidth to each MS according to the requirements of the incoming traffic without involving the MS. On the uplink, the MS may make requests for uplink bandwidth via a polling mechanism overseen by the BS.
The WiMAX MAC layer is connection-oriented in the sense that prior to data transmission, a logical link called a connection must be established between the BS and the MS. The connection is assigned a connection identifier (CID). The connection-oriented architecture allows WiMAX to support fine-grained Quality-of-Service (QoS). A service flow may be a unidirectional flow of packets associated with a set of QoS parameters and identified by a service flow identifier (SFID). The QoS parameters may include priority, maximum sustained traffic rate, minimum tolerable rate, maximum delay, etc.
WiMAX specifies five scheduling services summarized below:                1) Unsolicited grant services (UGS): This service supports constant bit rate (CBR) traffic with fixed-size data packets.        2) Real-time polling services (rtPS): This service supports real-time variable bit rate (VBR) traffic flows that generate variable-size data packets on a periodic basis.        3) Non-real-time polling services (nrtPS): This service supports delay-tolerant flows that require a minimum guaranteed traffic rate.        4) Best-effort service (BE): This service supports data streams that do not require minimum QoS guarantees.        5) Extended real-time variable rate (ERT-VR) service: This service supports real-time traffic flows that require a guaranteed data rate and delay.        
While WiMAX provides extensive bandwidth allocation and QoS mechanisms, it does not specify or standardize any details of scheduling and management.
C. Cellular Systems.
WiMAX systems may be deployed as cellular systems in a geographic coverage area partitioned into smaller regions called cells. Each cell may be served by a base station, which limits its transmit power to provide sufficient signal strength at the cell boundary. Propagation path loss allows base stations in spatially separated cells to transmit at the same carrier frequencies without causing harmful interference to each other.
In conventional cellular systems based on FDMA (Frequency Division Multiple Access), the system bandwidth may be divided into frequency channels of equal bandwidth. Each channel provides a communication link for a single connection or call. If frequency-division duplexing (FDD) is used, separate frequency channels may be allocated for the uplink and downlink channels. In time-division duplexing (TDD), a single frequency channel supports both the uplink and downlink channels via time-division multiplexing.
As discussed above, WiMAX is based on OFDMA, which allows an allocation of spectrum to users to accommodate different traffic types and data rate requirements. In OFDMA, subcarriers are grouped into subchannels which are allocated to users. From the user's perspective, an OFDMA subchannel corresponds to a frequency channel in conventional FDMA-based cellular systems, except that the bandwidth of an OFDMA subchannel can be variable. To avoid co-channel interference in OFDMA, however, the subcarrier may be the basic unit of frequency allocation.
Frequency allocation in cellular systems may be described in terms of frequency channels in conventional FDMA-based cellular systems with the understanding that for WiMAX, frequency allocation would be performed at the granularity of a subcarrier.
The mechanism used to assign frequency channels within a cell in an FDMA cellular system is referred to as a channel assignment scheme. Two channel assignment schemes that have been used in conventional cellular networks are fixed channel assignment (FCA) and dynamic channel assignment (DCA).                1) Fixed Channel Assignment (FCA): In FCA, the coverage area is partitioned into groups of contiguous cells called clusters. The set of frequency channels may be partitioned evenly among the cells in any given cluster such that each cell in the network is allocated a predetermined set of channels. Any call request within the cell can only be served by the unused channels assigned to that particular cell.        
To improve utilization, a borrowing option may be considered. With the borrowing option, a cell is allowed to borrow channels from a neighboring cell if all of its own channels are already occupied and the neighboring cell has spare channels. Borrowing is normally supervised by the mobile switching center (MSC). Since handoff is performed by the MSC, the MSC has full knowledge of the capacity usage of the cluster of cells within its jurisdiction. Therefore, the MSC is a subsystem that can oversee functions such as channel borrowing.                2) Dynamic Channel Assignment (DCA): In DCA, channels are not allocated to cells on a permanent basis. Each time a call request is made, the serving base station requests a channel from the MSC. The MSC dynamically determines the availability of a channel and executes its allocation procedure accordingly. The MSC generally only allocates a given frequency channel if that channel is not presently in use in the cell, or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference.        
The DCA scheme may be explained in terms of the cell cluster concept. For a given cell i, an associated MSC maintains a list of channels with an indication of whether the channel is free or occupied. When a call request arrives to cell i, the call is assigned a free channel, say channel c, if one is available. In this case, channel c is marked as “occupied” for all of the other cells in the cell cluster centered at cell i. Later, when the call completes, channel c is marked as “free” for all cells in the cluster centered at cell i.
DCA may reduce the likelihood of call blocking, which increases the trunking capacity of the system, since all available channels under control of the MSC are accessible to all of the cells. However, DCA schemes require the MSC to collect real-time data on channel occupancy, traffic distribution, and radio signal quality of all channels on a continuous basis. The MSC may need to do this data collection in order to manage handoffs between cells.